We can’t know they were the first. We can’t even know what the idea of a singular Sumerian culture or civilization was.
It’s fine to speculate, but it’s pointless to just decide that our speculation is fact when there is absurdly huge numbers of pre-historic people who may have stumbled upon writing, but never care about permanence.
There is absolutely nothing wrong with the proposition: “Ancient Sumerians probably created the first writing system”
Why? We have no records of other writing systems that have been conclusively dated to be earlier, and in this case (unlike others, e.g. Chinese) we DO have a very clear record of the development of the script, from accounting symbols to pictograms to ideograms to phonetic transcription. It definitely did not derive from an earlier script, as we see the progression of development in isolation.
So why can't we call it the first?
The emphasis is on 'know'. This is epistemology and logic. Do you know you are sitting on a chair? Yes. Do you know your birthday? No, in so far as you cannot personally verify it, as you do not recollect that time.
It is fine to assume all sorts, that your birthday is as your parents say, that sumerians are the oldest civilization, etc, but history, esp. ancient history is not knowledge, and can never be 'known'.
Using 'probably' indicates that this is one's best hypothesis, but doesn't overstate the case (nor mislead) by stating it as an indisputable, known fact.
Outside of a philosophy class, 'know' takes on quite a bit more pragmatic meaning which is perfectly appropriate here.
And inside philosophy class, I would argue, that the knowlege of the chair your are sitting on, is also not so much more confirmed and solid knowledge, than ones own birthday.
Outside philosophy class, do you not distinguish between those things you have personally experienced, characterising them as things that are known (to you), and between those things that you've seen on a screen or heard, characterising them as (mere) possibilities to you, yet to be experienced/verified?
I'd argue that it is pragmatic and useful to distinguish between 'know' and 'believe' (or 'hypothesise').
But yes, some people do use 'know', 'think', 'believe', 'feel', etc almost interchangeably. I think this cannot help but lead to confusion.